Method for molding plastic articles



Oct. 31, 1967 P, H. HOFER ET AL 3,350,487

METHOD FOR MOLDING PLASTIC ARTICLES Original Filed Aug 13, 1964" 2 Sheets-Sheet 1 PETER H.HOFER 44. HERBERT A SWALLOW ATTORNEY INVENTORS PETER H HOF HER I BERT .SW 0w By Wi 147 NE) United States Patent 3,350,487 METHOD FOR 'MOLDING PLASTIC ARTICLES Peter H. Hoifer, Berkeley Heights, and Herbert A. Swallow, North Branch, N.J., assignors to Union Carbide Corporation, a corporation of New York Original application Aug. 13, 1964, Ser. No. 389,254. Divided and this application Nov. 29, 1966, Ser. No.

1 Claim. (Cl. 264-161) This application is a devision of copending application Ser. No. 389,254, filed Aug. 13, 1964.

This invention relates to the melt compression molding of plastic articles. More particularly, this invention relates to an imprived method for melt compression moldng spru-free plastic articles. Even more particularly, this I to an improved method for melt compression molding sprue-free apertured plastic articles such as coaxial cable spacers.

elt compression through melt compression molding.

Another specific problem that has existed for some time relates to the molding of apertured plastic articles such mold cavities 22 melt compression molding coaxial cable spacers Within tolerable dimensional and property limits.

The present invention will be further understood from the following description taken in conjunction with the accompanying drawings wherein FIGURE 1 is a view partly in elevation and partly in section of apparatus suitable for carrying out the instant method for molding.

'FIGURE 2 is a perspective view of the bottom of the upper mold of the apparatus shown in FIGURE 1.

FIGURE 3 is a perspective view of the bottom of the piston of the apparatus shown in FIGURE 1.

FIGURE 4 is an enlarged view of a portion of the apparatus shown in FIGURE 1.

FIGURES 5 and 6 are views of the apparatus shown in FIGURE 1 showing the relative positions of the apparatus components dur g two stages of the molding cycle.

The method of this invention for molding a sprue-free plastic article broadly comprises supplying a metered Charge of molten plastic material to a transfer cylinder,

cooling the plastic in the absence of the mold cavity. If desired, a sprue-free plastic article can be molded having an aperture therein by As indicated above, the drawings illustrate apparatus which is adapted for producing coaxial cable spacers. It is for purposes of convenience only that the following dethe present invention.

Specifically referring to FIGURES lthrough 4, the preferred apparatus of this invention is shown to include a lower mold portion 10 having a recess 11 and a cylindrical passageway 12 adapted to receive a reciprocative aperture forming pin 13. Surmounting the lower mold portion 10 is an upper mold portion 14 having a transfer cylinder 16, a passageway 18 in communication therewith, and a sprue forming channel 20 in communication with the passageway 18 and a mold cavity 22 which is cooperatively defined by the mold portions 10 and 14.

As is more clearly shown in FIGURE 2, the mold por-- tions 10 and 14 can cooperatively define a plurality of all of which have a passageway 18 and a sprue forming channel 20 by means of which each of the mold cavities 22 is in communication with the transfer cylinder 16. In the drawings, the orifices 20" of the sprue forming channels are shown to be positioned olf center so as not to interfere with the aperture forming pins 13.

The passageway 18, more clearly shown in FIGURE 4, progressively decreases in cross section from the cylinder 16. This shape for the passageway 18 is conveniently employed to insure ease of separation of a cooled plastic material therefrom. The sprue forming channel 20 is shown in FIGURE 4 to be smaller than and to progressively decrease in cross section from the passageway 18 (that is the maximum diameter of channel 20 is less than the minimum diameter of passageway 18 as is clearly shown in FIGURE 4). The configuration for the sprue forming channel 20 has a zero land, that is, there is no portion constant in cross section. For this reason, problems such as elongation of plastic material when removed from a portion of constant cross section are eliminated. For purposes of the present invention, the sprue forming channel 20 is also sutficiently small to cause a molten plastic material flowed therethrough to undergo molecular shear and thereby generate frictional heat. This unique feature eliminates pellet outlines and half-moons heretofore encountered with prior apparatus for melt compression molding. Furthermore, this feature also provides for good color dispersion and complete fillout of the mold cavity.

The mold portions and 14 are conveniently provided with the internal conduits 9 and 17 respectively for purposes of circulating a cooling fluid therethrough to cool the mold portions.

Associated with the transfer cylinder 16 is transfer piston 24 having concentrically arranged conduits 26 and 27 for purposes of circulating a cooling fluid therethrough as indicated by the arrows in FIGURE 1. The face of piston 24 is also provided with a centrally located tapering dovetail slot 25 which extends across the bottom of the piston 24 as is more clearly shown in FIGURE 3. By means of the slot 25 excess cooled plastic material is removed from the upper mold portion 14 as is more fully described below.

The operation of the apparatus shown in the drawings will be briefly described with particular reference to FIGURES 1, 5 and 6. Polyethylene is fluxed in a suitable extruder (not shown) and is conveyed thereby to a suitable metering pump (not shown). From the pump, the molten polyethylene is fed through a nozzle and cut off by a suitable mechanism and supplied to the transfer cylinder 16 as molten charge 19 as shown in FIGURE 1. The piston 24 is then slid by suitable means into the transfer cylinder 16 and is employed to apply a primary force to cause the molten charge of polyethylene to flow and fill the mold cavities 22, the sprue forming channels 20, the passageway 18, a portion of the transfer cylinder 16 and the dovetail slot 25 of the piston 24. When this filling is complete, as shown in FIGURE 5, the polyethylene in the mold cavities 22 is cooled while a secondary force, generally less than the primary filling force, is simultaneously applied by the piston 24 such that the molten polyethylene 19 continues to flow due to molecular shear through the sprue forming channels 20 to compensate for shrinkage of the cooling polyethylene in the mold cavities 22. In this manner a molded article which is a true reproduction of the mold cavity is insured. After shrinkage of the polyethylene in the mold cavities 22 is substantially completed, application of the secondary force by the piston 24 is terminated and the polyethylene in the sprue forming channels 20 is allowed to cool in the absence of positive pressure. Because the polyethylene in the sprue forming channels 20 is surrounded by the cool mold portion 14, sprues are formed before the polyethylene immediately underlying the sprues is completely cooled. This is due to the fact that the polyethylene underlying the sprues is more remote from the cooling mold portion 14 and hence will cool more slowly than the polyethylene in the sprue forming channels 20. The method of the present in e t n akes advantage of this phenomenon to produce a sprue-free spacer by separating the cooled sprues from the polyethylene in the mold cavities 22 before the polyethylene underlying the sprue is cooled. Since the uncooled polyethylene is weak compared to the cooled sprues, separation is accomplished with ease. The separation can take place at the exact upper surface of the mold cavities 22 but in practice when the sprues are separated from the polyethylene in the mold cavities 22, a small amount of polyethylene is separated with the sprues leaving a minute depression in the surface of the spacers which in no way is a dimensional defect in the spacer.

Polyethylene in the dovetail slot 25 is cooled by the piston 24 and provides the means whereby the above described sprue separation is accomplished when the piston 24 is withdrawn from the transfer cylinder 16. This is shown in FIGURE 6. The excess polyethylene or cullet 19 removed by the piston 24 is subsequently removed therefrom by a force applied to the edge of the cullet 19' which slides the same out of the dovetail slot 25. The cullet 19 can then be recycled to the extruder as desired.

For the purposes of this invention it is preferred that the center line of the tapering dovetail slot 25 coincide with a diameter line of the transfer piston 24 as is illustrated in FIGURE 3. When the slot 25 is so positioned, the cooling polyethylene will shrink toward this center line which will eliminate any binding between the polyethylene and the slot 25. In practice, that portion of cullet 19 in the slot 25 will be slightly smaller than the slot 25 because of shrinkage on cooling and consequently the cullet 19 can be removed from the slot with ease. It should also be understood that more than one slot positioned and shaped in such a manner so as to prevent binding on cooling can be employed.

After the piston 24 is retracted removing therewith excess polyethylene 19', the mold portions 10 and 14 are separated by suitable means, the aperture forming pins 13 are retracted by suitable means and the finished spacers 21 are removed conveniently by a blast of air from nozzle 28. This is shown in FIGURE 6. Because molten polyethylene will shrink towards the center when cooled, polyethylene in the mold cavities 22 will shrink onto the aperture forming pin 13. This behavior can best be utilized by first raising the upper mold portion 14 as shown in FIGURE 6. Because the spacers 21 have shrunk onto the pins 13, they are held in place and will separate with case from the mold cavities 22. The pins 13 can then be retracted and the spacers 21 removed.

The ratio of the area of the face of the transfer piston to the area of the molded articles can range from about 4:1 to about 121.9 but for most all practical purposes it is in the range of from about 1:1 to about 111.9. The latter range is preferred for most applications since it provides a means of keeping waste down to a minimum and allows for the use of high forming pressures and hence more efficient operating rates.

From the foregoing, it will be obvious that the components of the apparatus shown in the drawings can be used in a molding cycle which comprises a coordinated series of timed operations. As such the apparatus of this invention is ideally suited for high-speed, eflicient production of molded articles, sprue-free holded articles, and sprue-free apertured molded articles such as coaxial cable spacers and the like.

Several modifications of the apparatus shown in the drawings can be mentioned as being representative of modifications and changes which can be made within the scope of this invention. For example, the upper mold portion 14 may rest under the force of gravity on lower mold portion 10 or may be clamped thereon during the molding cycle. Also, suitable guide means may be utilized between the mold portion 10 and 14 and for the piston 24 to insure proper alignment.

The manner in which the mold portions 10 and 14 cooperate to define the mold cavities 22 may vary from that of the mold cavities.

As will be evident to those skilled in the art, the mold s 22 and the transfer piston 24 can be vented to thereby facilitating removal of the cullet 19".

thermoplastic resins which will flow Apparatus similar to that shown in the drawings was used but having only one mold cavity and without the manner an excellent quality flashlight lens free let outlines and half-moons was produced. The above procedure was also followed to produce star shaped articles from polystyrene and polyethylene.

Example 2 Apparatus similar to that shown in the drawings was used to mold sprue-free coxial cable spacers. The ratio of the area of the face of the transfer piston to the area 6 of the molded spacers was 1:13. Polyethylene having melt index of 0.2-0.4 gm./ 10 min. and a density of .91

.922 gm./cc. was fluxed and conveyed in a 1 /2" Mode FIGURES 1, 5 and 6 the following conditions.

Temperature of molten polyethylene C 14 Extruder back pressure p.s.i 180 Manifold back pressure p.s.i Mold temperature C 11 Transfer piston temperature C 1 Primary filling pressure p.s.i 400( Secondary filling pressure p.s.i 500( Duration of primary pressure second 0.08 Duration of secondary pressure and sprue cooling in absence of positive pressure do 1.02 Total molding cycle do 1.10 Air nozzle discharge pressure p.s.i 80 Duration of discharge second 0.7

Spot checks throughout the 50 minute run indicated that the spacers were sprue free and without flash and otherwise satisfied dimensional and property specifications.

We claim:

sprue is cooled and removing the sprue-free molded article so formed from References Cited UNITED STATES PATENTS 2,355,613 8/1944 Wacker. 3,071,814 1/1963 Guhhenheim 264329 3,174,187 3/1965 Schriever 264 l61 3,265,797 8/1966 Spaak 264328 ROY B- M FFIIT; m y a iner 

